The present invention relates to a vehicle seat belt retractor, and particularly relates to a vehicle seat belt retractor having a mechanism for reducing tension in seat belt webbing.
A known vehicle seat belt retractor comprises a spool for holding a wound length of seat belt webbing. The spool is supported for rotation in belt withdrawal and belt retraction directions. A spring biases the spool to rotate in the belt retraction direction to wind up the webbing onto the spool. The spool rotates in the belt withdrawal direction against the force of the spring when the webbing is unwound from the spool to be buckled around a vehicle occupant. After the webbing is buckled around the vehicle occupant, slack in the webbing is wound onto the spool because the spool is rotated in the winding direction by the spring. The webbing is thereafter held against the vehicle occupant by the force of the spring biasing the spool to rotate in the belt retraction direction. The force of the spring biasing the spool in the belt retraction direction and holding the webbing against the vehicle occupant may cause discomfort to the occupant.
Mechanisms for minimizing the force of the spring biasing the spool and holding the seat belt webbing against the occupant are known. U.S. Pat. No. 4,301,977 discloses a retractor having a spring with one end connected to a retractor housing and its other end connected to a disk rotatably supporting a plurality of planet gears. The planet gears mesh with a sun gear that is attached to a retractor spool around which a seat belt is wound. The planet gears also mesh with a ring gear that is rotatable relative to the retractor spool. Axially extending studs rotatably support the planet gears and extend into arcuate slots connected with the ring gear. A rocker block associated with the ring gear engages a fixed surface of the housing to prevent the ring gear from moving in the belt retraction direction.
Withdrawal of the belt causes the sun gear to rotate in the belt withdrawal direction which urges the planet gears to rotate in the opposite direction, which, in turn, produces a force in the belt retraction direction against the ring gear. Since the ring gear is prevented from rotating in the belt retraction direction, the engagement of the planetary gears with the ring gear causes the planetary gear studs to move in the belt withdrawal direction. The studs engage the ends of the slots in the ring gear and overcome the force of the rocker block to rotate the ring gear in the belt withdrawal direction. Thus, the sun gear, the disk and the ring gear all move in the belt withdrawal direction.
Upon buckling of the seat belt, the spring acts through the planet gears and sun gear on the spool of the retractor in a low tension mode. The spring urges the planetary gear studs to move in the belt retraction direction in the arcuate slots in the ring gear. The rocker block holds the ring gear stationary, causing the planetary gears to rotate in a counterclockwise direction about their axes which causes the sun gear to rotate in the belt retraction direction. Thus, the retractor is in a low tension mode due to the gear reduction between the planet gears and the sun gear.
When the seat belt is unbuckled, the planetary gear studs move in the belt retraction direction until they engage the ends of the arcuate slots in the ring gear. One of the planetary gear studs engages the rocker block to pivot the rocker block in a direction which allows the ring gear to move in the belt retraction direction. Thus, the sun gear, the disk and the ring gear all move in the belt retraction direction to retract the belt onto the spool.